Number of planets in our solar system is up in the air

Pluto — long considered the most distant and the smallest of the solar system’s planets — in now classified as a dwarf planet. / Illustration/Getty Images

From 1930 to 2006, our solar system had nine planets.Now it has eight.

What happened? Did a planet explode?

No, it was not anything that dramatic. It’s just that Pluto — long considered the most distant and the smallest of the solar system’s planets — was reclassified by the International Astronomical Union. Its name was erased from the list of official planets and, instead, it became what is known as a dwarf planet.

The significant difference between a planet, like Earth, and a dwarf planet, is that the dwarf planet does not have enough gravitational pull to clear its orbit of other objects (either by pulling them in or scattering them away). Pluto, for instance, orbits in what is known as the Kuiper Belt, a region beyond Neptune made up of thousands of icy bodies known as Kuiper Belt objects, or KBOs. They are also sometimes referred to as transneptunian objects.

Five and counting

However, not just any old space rock can be a dwarf planet. To qualify, it must be orbiting the sun, it must have enough mass to take a primarily round shape, and it cannot be the moon of another object.